Air Traffic Control (ATC) depends upon secure and reliable communications between ground-based controllers and aircraft in controlled and non-controlled airspace. In order to meet the needs of secure and reliable communication, the Aircraft Communications Addressing and Reporting System (ACARS) protocol and the Aeronautical Telecommunications Network Open Systems Interconnection (ATN OSI) protocol were designed in the ATC data communication protocol stacks, to name a few. The ACARS protocol utilizes various sub-networks, such as Very High Frequency Data Link-Mode 2 (VDL-Mode 2), Plain Old ACARS (POA or VDL Mode 0/A), ACARS over AVLC, Inmarsat (SATCOM), Iridium (SATCOM), and High Frequency Data Link (HFDL), that may be used to transmit the messages. Currently, the United States uses the ACARS based protocol, first deployed in 1978. Although there are many types of ACARS protocols available, the United States specifically uses the Future Air Navigation System (FANS) protocol. The FANS protocol provides a direct data communication link between the pilot in the aircraft and the air traffic controller at the ground-based controller.
In Europe, ATC data communication uses Link2000+, a type of ATN OSI protocol also known as ATN B1 implementation using solely the VDL Mode 2 sub-network. The International Civil Aviation Organization (ICAO) introduced the ATN OSI protocol around the year 2000, following the certification of FANS in 1995. There was a general belief that the newer ATN OSI was superior to the older FANS in terms of performance due to the advancements in technology. In response, the Federal Aviation Administration declared the FANS protocol would transition to the ATN OSI protocol in the United States. The FAA decided to implement this transition and wrote in the FAA Data Communications Program (DCP) documents comprising the Segment 1 (S1) Investment Analysis Readiness Decision (IARD), the S1 Initial Investment Decision (IID) and the S1 Final Investment Decision (FID) information pertaining to this transition. However, the assumption that ATN OSI would be superior to FANS has proved to be premature.
Today, ATN OSI over VDL Mode 2 is experiencing technical issues in Europe including provider aborts and long delays.
Provider aborts occur in a communication system when there may be a sustained loss of end-to-end connectivity, thus loss of availability. Even though these provider abort and long delay issues were observed some time ago, they did not receive substantial attention until relatively recently.
European Technical groups were formed to investigate the technical issues with the ATN OSI European data link. In 2014, the European Aviation Safety Agency (EASA) released an investigation report on technical issues in the implementation of a European Rule of the ATN OSI over VDL Mode 2. EASA found the technical issues to be sufficiently critical that the European Commission decided to postpone the ATN OSI data link rule for 5 years, from 2015 to 2020. Given the complicated nature of the ATN OSI over VDL Mode 2 technical issues, whether they can be completely fixed or not is an open issue.
In addition to ATN OSI technical issues, the FANS customized protocol also suffers from delays associated with latency performance. In the ATN OSI network, provider aborts take place after a delay of longer than 6 minutes. In the FANS network, there is no provider abort mechanism, so FANS does not have a provider abort issue, however the FANS protocol employs a means to try other sub-networks, in series. FANS, therefore allows longer delays, which suits strategic planning but which makes it impractical for tactical operation. The performance issues in FANS and ATN OSI prevent them from replacing voice to become a primary ATC communication for time critical exchanges. Accordingly, there is a need for improvement in air traffic control communications availability and latency performance.
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